Several microbiological particle control systems exist. Mechanical and electronic filters can be used to reduce indoor concentrations of respirable particles, such as in a forced air heating/cooling system of a building. Microbiological filters have been used for disinfection of air and other gases because of their low cost and ease of handling. These filters can be constructed to remove not only microorganisms, but also submicron particles as well. For efficient and economic operation of these filters, the aerosol content of the air to be filtered must be low, where the term “aerosols” generally refers to microorganisms, particles, and droplets of liquid dispersed in air. A disadvantage of such filters is that they do not permanently remove the contaminants, but just transfer them to the filter medium. Clogging of the filter medium can result which can cause high pressure drops. In addition, microorganisms trapped on the filter media continue to multiply making the filter media a breeding ground and thus hazardous.
Another method of microbiological particle removal is UV disinfection. UV disinfection has been widely used to destroy biological contaminants and toxic chemicals. Such UV treatment has worked well for disinfection, but the indoor environment may also be contaminated with low level toxic chemicals such as formaldehyde, styrene, and toluene. Ultraviolet energy alone has proven generally ineffective in degrading these chemicals.
Another alternative that has gained increasing attention is photocatalytic oxidation (PCO), which involves the use of a photocatalyst such as TiO2 for the destruction of hydrocarbons and microorganisms in fluids. Titanium dioxide (TiO2) is a semiconductor photocatalyst with a room temperature band gap energy of about 3.2 eV. When this material is irradiated with photons having wavelengths less than about 385 nm (UV), the band gap energy is exceeded and electrons are generated through promotion from the valence band to the conduction band which results in the generation of holes (h+). The resulting highly reactive electron-hole pairs have lifetimes in the space-charge region of the photocatalyst that enables participation in chemical reactions, provided recombination events do not occur first. The most widely postulated chemical reactions are shown below:OH−+h+→OH (hydroxyl radical)O2 +e−→O2−(super-oxide ion)
Hydroxyl radicals and super-oxide ions are highly reactive species that can readily oxidize volatile organic compounds (VOCs) adsorbed on catalyst surfaces. They can also kill and oxidize adsorbed bioaerosols. The process is a form of heterogeneous photocatalysis, or more specifically PCO.
Several attributes of PCO make it a strong candidate for indoor air quality applications. Pollutants, particularly VOCs, are preferentially adsorbed on photocatalytic surfaces and oxidized primarily to carbon dioxide (CO2). Thus, rather than simply changing the phase and concentrating the contaminant, the absolute toxicity of the treated air stream is reduced, allowing the photocatalytic reactor to operate as a self-cleaning filter.
Photocatalytic reactors may be integrated into new and existing heating, ventilation, and air conditioning (HVAC) systems due to their modular design, room temperature operation, and generally negligible pressure drop. These attributes contribute to the potential of PCO technology to be an effective process for removing and destroying low level pollutants in indoor air, including bacteria, viruses and fungi.
For example, U.S. Pat. No. 5,933,702 Goswami discloses a method for disinfecting an air stream containing microorganisms. The method includes the steps of providing an air stream containing microorganisms having a relative humidity greater than about 40% and contacting the air stream with a photocatalyst having a predetermined band gap energy in the presence of a source of photons having a wavelength corresponding to the band gap energy of the photocatalyst. At least a portion of the microorganisms in the air stream are destroyed by photocatalytic oxidation. Improved devices embodying the method of the invention are disclosed, such as stand-alone devices and devices incorporated into the HVAC systems of buildings, including the air supply registers. Photocatalyst-coated filter media capable of trapping bioaerosols are also disclosed.
The photocatalytic process is energy intensive because of the very low absorption rate of light by the photocatalyst. Accordingly, the light radiation flux required at the photocatalyst surface needs to high and most of the light energy is not utilized for the disinfection process.
Further, only the exterior surface of the photocatalyst, participates in the destruction processes. This area could be much smaller than area of the substrate on which the photocatalyst is deposited. The low light absorption and low disinfection area leads to a low destruction rate and high power consumption for the photocatalytic disinfection process. The low destruction rate also results into slow regeneration of the fouled photocatalyst. In addition, photocatalytic processes are made further inefficient due to hole-electron recombination of a portion of the photogenerated holes and electrons.